The present invention relates to thermoplastic polyester alloys usable particularly for the production of moulded articles strengthened against impact.
The impact strengthening of thermoplastic polyesters such as, in particular, polyethylene terephthalate and polybutylene terephthalate constitute a continual preoccupation of the industry producing articles made of these materials. This preoccupation concerns not only strengthening against impact at ambient temperature but, increasingly, also strengthening against impact at very low temperatures. In fact, such moulded articles are likely to be employed not only for domestic purposes but also outdoors in cold countries, for example in the case of car components.
Thus U.S. Pat. No. 4,172,859 describes in particular a tough multiphase thermoplastic composition consisting essentially of a phase containing from 60 to 99% by weight of a saturated polyester matrix and from 1 to 40% by weight of another phase containing particles of a random copolymer from 0.01 to 3 microns in size, the said random copolymer being based on ethylene, on a first comonomer chosen from the anhydrides of unsaturated dicarboxylic acids and unsaturated epoxides and on a second comonomer chosen from acrylic and vinyl esters. Among the embodiments of this document which involve a random copolymer comprising an unsaturated epoxide, there may be noted a composition with a notched Izod impact strength of up to 86 kg cm/cm at 20.degree. C. (value measured at the end of the specimen away from the injection side), this composition comprising 20% of a terpolymer containing 67% by weight of ethylene, 5% by weight of glycidyl methacrylate and 28% by weight of vinyl acetate. It should be noted, however, that the presence of vinyl acetate in this terpolymer constitutes a disadvantage when this composition is being converted; in fact, a release of acetic acid, reflecting a degradation of the terpolymer, is produced at the usual temperature of conversion into industrial articles.
Other documents show the possibility of improving the low-temperature impact strength of thermoplastic polyesters. Thus, patent EP-A-072,455 describes a composition comprising 100 parts by weight of polybutylene terephthalate, 10 parts by weight of a copolymer comprising 90% by weight of ethylene and 10% by weight of glycidyl methacrylate, and 20 parts by weight of a copolymer comprising 85 mol % of ethylene and 15 mol % of 1-butene. This composition has a notched Izod impact strength of 19 kg cm/cm at -40.degree. C. Published Japanese Patent Application No. 86-204,258 describes a composition comprising 80 parts by weight of polybutylene terephthalate, 10 parts by weight of a copolymer consisting of 89% by weight of ethylene and 11% by weight of glycidyl methacrylate, 10 parts by weight of a copolymer consisting of 91 mol % of ethylene and 9 mol % of 1-butene, and of 1 part by weight of maleic anhydride. This composition has a notched Izod impact strength of 19 kg cm/cm at -30.degree. C.
It should be noted that, while the low temperature performance levels achieved by the compositions of these last two documents already contribute a substantial improvement in the low-temperature performance of unmodified polybutylene terephthalate, they are still inadequate for many applications. In addition, these compositions exhibit ambient temperature performance which is clearly inferior to those described originally in the case of the U.S. Pat. No. 4,172,859. As a result, a first objective of the present invention consists in developing thermoplastic compositions based on saturated polyesters, having an excellent impact strength both at ambient temperature and at very low temperature. A second objective of the present invention, consists in developing thermoplastic compositions based on saturated polyesters not exhibiting the disadvantage of a decomposition of one of its constituents under the high temperature conditions to which the said compositions are subjected while being converted into industrial articles. A third objective of the present invention, consists in developing thermoplastic compositions based on saturated polyesters which, in addition to the above advantages, have a viscosity which is adapted to their conversion by injection moulding, and more particularly a melt index (determined at 250.degree. C. under a load of 5 kg) of at least 10 dg/min.